The present invention relates to a programmable sequence controller, and more particularly to the facilitated addressing of a programmable sequence controller for a large number of input/output circuits constructed as units.
A conventional "programmable sequence controller system" (hereinbelow termed "PSC") is constructed of a central processing unit which principally executes logic operations, a program memory which stores the contents of programmable logic operations, an input circuit which receives the signals of external contacts in accordance with the arithmetic processing portion of the central processing unit on the basis of the content of the program memory, and an output circuit which delivers an operated result so as to drive external equipment.
With the conventional PSC thus constructed, when the control scale thereof becomes large, the number of input/output circuits separately constructed and arranged increases naturally. In this regard, addresses are selected by one of several methods.
In these methods, input and output circuit portions (the so-called unit type input and output circuits) are disposed separately from the arithmetic control unit which includes a central processing unit and the program memory. In one of these methods, addresses defined by switches for the respective input and output units are appointed by signals from the arithmetic control unit. In another method, addressing signal lines are successively shifted at points inside the respective input and output units whereby addresses are automatically appointed in hardware fashion.
The above methods, however, have the disadvantage of requiring a large number of address signal lines which interconnect the input/output circuit units and the arithmetic control unit. Another problem is that the system is liable to receive much external noise through the signal lines. Moreover, in arranging the input and output units, these units need to be distinguished. This leads to such disadvantages that a special circuit therefor must be used and that signals to be transmitted must be individually identifiable in advance.